Dexamethasone and spironolactone in rats induce the same cytochrome P-450, (P-450p) as does pregnenolone 16 Alpha-carbonitrile. Cytochrome P-450p possesses a high ethylmorphine N-demethylase activity in intact microsomes. This N-demethylase activity was blocked by antibody raised against cytochrome b5 comparable to the degree of inhibition of cytochrome c reduction mediated by cytochrome b5 and NADH cytochrome b5 and NADH cytochrome b5 reductase. Likewise these microsomes displayed regioselective induction of hydroxylation at 6 Beta-position of both testosterone and progesterone at the expense of hydroxylation of 16 Alpha-position, suggesting that there is a specific cytochrome P-450 responsible for the hydroxylation. By analogy with the N-demethylase, the antibody against cytochrome b5 blocked preferentially the hydroxylation at the 6 Beta position of the steroids. In addition it also blocked the formation of an unidentified metabolite of testosterone.